1. Field of the Invention
This invention relates to floating covers of the pontoon type, and, in particular, to internal floating covers wherein one of two next-longitudinally adjacent pontoons is pivotally mounted and rotatable with respect to the longitudinal axis of the other.
2. Description of the Prior Art
Experience has shown that of the various efforts designed to minimize the loss of liquid products from fixed-roof storage tanks, the simplest and most economical solution is an internal cover flotationally disposed over the exposed liquid surface. Initially, various floating covers were utilized, fabricated of polyvinyl chloride, polyesters, epoxies, plastic and plastic foams, steel and aluminum. Some covers floated directly on the product while other constructions supported the covers over the product on suitable flotation devices.
Three basic types of internal covers have been developed. These were steel pan-type covers, plastic foam raft-type covers, and aluminum pontoon-type covers.
Believed exemplary of the steel pan-type covers are the devices disclosed in U.S. Pat. Nos. 3,511,406 (Creith et al.), 3,972,444 (Adams), 3,944,113 (Heisterberg), 4,036,394 (Bodley et al.), and 4,071,164 (Skakunov). The Skakunov device includes a resilient disc-shaped diaphragm resting upon a series of radially disposed telescopic arms each attached at one end to a central floating chamber and at the other end to an annular seal. The seal end of each arm is provided with a fork which is secured by a fulcrum pin to a roller to thereby effect a resilient connection to the seal.
However, due to the problems of sinking, low efficiency, and high initial cost, steel pan-type internal covers are believed to have declined in favor among users of internal covers.
Annular seal arrangements are necessary in most, if not all, internal covers (of any type) of effect a vapor seal between the periphery of the cover and the interior of the usually cylindrical tank in which they are disposed. Believed typical of such seal arrangements are U.S. Pat. Nos. 3,343,708 (Hass), 3,372,831 (Daniels et al.), 3,926,332 (Okamoto), 4,036,395 (Turkey), and 4,116,358 (Kinghorn et al.).
U.S. Pat. Nos. 3,910,452 (Szasz) and 4,018,356 (Szasz et al.) are believed exemplary of the plastic foam raft-type internal cover. Due to their tendency toward delamination, product absorption, and static sparking potential, such plastic foam raft-type covers are also believed to be falling into disfavor among users of such covers.
In view of the foregoing, it is now believed that the most effective internal cover in use is the aluminum pontoon type floating cover. One such floating cover, sold by Ultraflote Corporation under the registered trademark ULTRAFLOTE, is disclosed and claimed in U.S. Pat. No. Re. 29,270 (Nelson), a reissue of U.S. Pat. No. 3,861,555. U.S. Pat. No. 3,942,674 (also to Nelson) relates to an electrical grounding arrangement for an internal floating cover.
Internal covers typically provide downwardly depending legs or struts for the purpose of supporting the cover above the bottom of the tank when the liquid product is withdrawn therefrom. Examples of the use of such depending support legs in a foam raft-type cover and in an aluminum pontoon-type cover are believed to be respectively disclosed in U.S. Pat. No. 3,910,452 (Szasz) and in U.S. Pat. No. Re. 29,270 (Nelson), both alluded to above.
It is believed that all manufacturers of aluminum pontoon-type covers utilize a rigid joint between longitudinally adjacent pontoons and fixedly support the depending legs at these locations. The rigid connection is utilized because it is the easiest to fabricate and properly install. Moreover, since the pontoons are the strongest portion of the cover and function as the main structural beams, it is most efficient to utilize a rigid joint therebetween and to attach the legs at that joint.
In other instances, as set forth in the referenced U.S. Pat. No. Re. 29,270, it is advantageous to have the capability of varying the length of the depending legs. Thus, the adjustable leg may be received within a sleeve attached through the deck. The sleeve is held securely to brackets which rigidly join adjacent longitudinal ends of pontoons together. The length of the leg may be adjusted from above the deck, through the sleeve.
Structurally, a rigid joint at the interconnection of the ends of longitudinally adjacent pontoons makes the pontoons act as a continuous beam. As is well known, a continuous beam is stronger than a series of simple beams, but, under uniform load, approximately twice the moment is imposed at the connections than at the midpoint of the spans. This fact has been outweighed by the perceived simplicity of a rigid joint.
It has been observed that high turbulence in a localized portion of the liquid has a tendency to raise and lower the cover unevenly, causing it to flex in some cases more than would be expected. The general background of aluminum internal floating roofs, as well as an explanation of the causes of various turbulent forces which are generated in the liquid product, is set forth in a paper, "Aluminum Internal Floating Roof", MC-78-6, by Ronald Carl Kern, presented at the Refinery and Petrochemical Plant Maintenance Conference, Feb. 8-10, 1978. Although aluminum pontoon-type internal floating covers inherently have some flexibility associated therewith as a result of their construction, especially due to the bending of the metal in the end plates of the pontoons, it can be shown that the turbulence may be of such severity that the end plate connected to each end of a pontoon may rupture after repeated flexings. Thus, under cyclic loading generated by severe turbulence, the welded joints at the ends of adjacent pontoons may break. This, of course, is disadvantageous.
In view of the foregoing, it is believed to be advantageous to provide a floating cover for internal or external use wherein the confronting ends of longitudinally adjacent flotation pontoons are pivotally mounted such that one pontoon is relatively movable with respect to the longitudinal axis of the other and that one pontoon may pivot with respect to the other as the cover flexes in response to turbulence in the liquid product or other non-uniform loading on the cover.
It is believed to be of further advantage to provide a floating cover wherein either fixed-length or adjustable-length support legs may be rigidly connected to and supported from one of two confronting ends of longitudinally adjacent pontoons with the end of the other pontoon being pivotally connected to the leg-first pontoon interconnection such that the second pontoon is pivotal relative to the longitudinal axis of the first pontoon. In this manner, it is believed to be advantageous to define a degree of relative pivotal rotative motion between longitudinally adjacent pontoons to thereby allow uplifting of the cover due to turbulence or to other non-uniform loading on the cover to cause flexure at the pivotal joint without stressing the welded end plate-pontoon connections. As a further result, each pontoon is permitted to act as a simple beam when the cover is supported by the legs without any moment in the end connection.